Stab at one by going mad (7)
I believe the answer is:
bayonet
'stab' is the definition.
(bayoneting is a kind of stabbing)
'at one by going mad' is the wordplay.
'going mad' indicates anagramming the letters.
'at'+'one'+'by'='atoneby'
'atoneby' with letters rearranged gives 'BAYONET'.
(Other definitions for bayonet that I've seen before include "Stabbing weapon" , "one's attached to one's rifle - that's sort of fitting" , "Pointed weapon" , "Type of light bulb fitting" , "Muzzle blade" .)